CMOS imaging arrays are utilized in numerous digital cameras. The imaging arrays are constructed from arrays of pixel cells in which each cell typically includes a photodiode and amplification circuitry for converting the photons accumulated during an exposure to a voltage signal that is then digitized to generate a picture.
The imaging arrays utilized in cameras that are designed for daytime photography differ significantly from imaging arrays that are designed for night or low-light photography, and hence, two different cameras must often be purchased and installed if images from both day and night are to be recorded. For daytime applications, the imaging arrays typically are constructed from pixels that have selective color sensitivities and block infrared wavelengths. For example, one commonly used design has clusters of pixels, one red, one blue, and two green sensitive pixels and an infrared filter that removes long wavelength radiation from the image before the photons reach the imaging array. Each pixel is constructed from a photodiode that is covered by a pigment filter that provides the different color sensitivity. The pixels in the green region of the spectrum have the highest sensitivity and are utilized for computing the overall light intensity as well as the intensity of light in the green region of the spectrum. For reasons related to the subsequent compression of the image, each cluster utilizes two green pixels.
Night imaging arrays typically use only one type of pixel and lack the infrared blocking filter. Since the number of photons that are available at each pixel is limited, the night cameras typically use larger pixels and lack wavelength filters that would further reduce the number of photons available at each pixel. The illumination that is available at night is typically shifted to the long wavelengths, and hence, the color rendering index of the illumination source is poor. Accordingly, providing color sensitive pixels is of significantly less value in night cameras.
Both types of cameras suffer from limited dynamic ranges. In general, each pixel has a range of light exposures that the pixel can measure. At low light exposures, noise dominates the measurement. At high light exposures, the pixel saturates, and hence, the output signal is no longer related to the light level. The dynamic range of typical commercial cameras is typically of the order of a 1000. That is, the exposure at which the pixel saturates is 1000 times the minimum exposure at which the pixel can detect an exposure over the noise levels.
If a scene does not include regions that differ in intensity by more than the dynamic range available from the imaging array, a satisfactory image can usually be recorded by selecting the correct overall exposure for the image. The exposure is set by adjusting the lens aperture and the exposure time. Cameras typically sample the scene to determine the best exposure. If the required dynamic range is greater than that available from the imaging array, or if the wrong sample points are chosen, this strategy fails, and at least part of the image will be under exposed or over exposed.
Even in cases in which the range of intensity values in the scene fits into the dynamic range of the camera, exposure problems can still be significant. Consider an image in which the exposure is set so that the bright portions of the image generate digital values that are near the maximum count of the analog-to-digital converter used to digitize the output of the photodiodes. Assume that this count is 1000. The error introduced by the digitization is ±½ count of the analog-to-digital converter, since all signals are rounded to the nearest count. This error is equivalent to introducing noise having a magnitude of ±½ count into the picture, and hence, will be referred to as the digitization noise in the following discussion. Consider two pixels, one having an exposure that produces a digital value of 500 and the other having an exposure that produces a digital value of 5. The later has a digitization noise of 10%, which is detectable by the human eye, while the former has digitization noise of 0.1%, which is not detectable by the eye. Hence, the low light portions of the image present problems even in the case in which the entire scene fits within the dynamic range of the camera.
In principle, the exposure problem can be improved by increasing the dynamic range of the pixels. However, this solution substantially increases the cost of the imaging array. To reduce the digitization noise, analog-to-digital converters that have significantly higher counts are needed, which increases the cost of the cameras.